1. Field of the Invention
This invention relates to a delay circuit using an A/D converter, a memory, and a D/A converter used to produce a surround sound of an audio machine, etc.
2. Description of the Related Art
Hitherto, a surround sound of an audio machine has been produced by delaying a reproduced sound for a predetermined time, attenuating, and superposing the resulting sound. Such reproduction modes as a stadium mode and a church mode adapt different manners of delaying and superposing reproduced sounds.
One of the delay circuits used for such purposes is shown in FIG. 1. In the circuit, an analog reproduction signal is temporarily converted into digital data by an A/D converter 10 and the digital data is stored in a memory 12. The data read from the memory 12 is converted back into analog data by a D/A converter 14. In the circuit, the delay of the read timing from the write timing is varied and the difference therebetween becomes a delay time.
Used as the A/D converter 10 is one as shown in FIG. 2. The A/D converter 10 comprises an adder 20, a quantizer 22, a variable integrator 24, and a time constant controller 26; an analog output of the variable integrator 24 is fed back into the adder 20 to which an analog signal is input. The quantizer 22 comprises a comparator 22a which outputs high or low in response to input signal voltage and a latch circuit 22b which latches the output of the comparator 22a in response to a predetermined clock for converting an input signal into a 1-bit digital signal. The variable integrator 24 integrates outputs of the quantizer 22 for providing an analog signal corresponding to the input signal. The output of the variable integrator 24 is fed back into the adder 20, which then finds a difference between both the signals; for the input signal with no change, the quantizer 22 outputs high and low alternately.
On the other hand, the controller 26 changes a time constant in the variable integrator 24 in response to the output condition of the quantizer 22. That is, if the output level of the adder 20 is large and the quantizer 22 makes an unbalanced output between "0" and "1," the time constant of the variable integrator 24 is made small; if the output level of the adder 20 is small and the quantizer 22 outputs "0" and "1" keeping a balance therebetween, the time constant of the variable integrator 24 is made large. Under such control, when there is no sound, the time constant can be made large to suppress occurrence of high-frequency noise; when the input signal changes, the time constant can be made small to output a high tone sufficiently.
On the other hand, the D/A converter 14 comprises a latch circuit 27, a variable integrator 28, and a time constant controller 29, as shown in FIG. 3. A pulse-string input signal read from the memory 12 is latched in the latch circuit 27, then integrated by the variable integrator 28 to provide an analog signal. Here, a time constant in the variable integrator 28 is controlled by a signal from the time constant controller 29 which has the same configuration as the time constant controller 26 shown in FIG. 2. That is, the time constant controller 29 detects the level of an input signal and controls the time constant of the variable integrator 28 accordingly. Since the input and output signals of the memory 12 are basically the same, such control enables the time constant of the variable integrator 28 to match with that of the variable integrator 24 for delaying the same signal as the input signal for a predetermined time for output.
However, in such a conventional delay circuit, the conversion characteristics of the A/D converter 10 cannot match those of the D/A converter 14 completely, causing a level change, etc., to occur, leading to a mismatch between the input and output analog signals. That is, generally, analog circuits such as CR low-pass filters are used as the time constant controllers 26 and 29 and the characteristics of these circuits change due to temperature changes, etc. Thus, it is difficult to completely match the characteristics between the two time constant controllers 26 and 29; a mismatch occurs in signals.
If the delay time is changed to alter the reproduction mode in the conventional delay circuit, the read timing from the memory 12 will be changed. If the read timing is simply changed, control of the A/D converter 10 and that of the D/A converter 14 cannot be made the same and the delay signal becomes different from the input signal, making the listener feel a sense of acoustic disorder. Then, to change the reproduction mode, the memory 12 may be initialized with its output disabled. However, such initialization of the memory requires a circuit configuration to initialize the memory 12.